1. Field of the Invention
The present invention relates to a threshold matrix generation device and a method for generating a threshold matrix for generating a threshold matrix of a predetermined size used for halftone processing of a multitone image data using a dithering method.
2. Description of Related Art
A FM (Frequency Modulation) screen is known as an expressing method of intermediate tones for an image formation. And a screening process (half-toning process) by a dithering method using a threshold matrix is known to perform the FM screening. The process does quantization of an image data by arranging threshold (values) matrix corresponding to the multi-toned image data like tiles and outputting results of comparison between the image data and the threshold matrix.
With regard to a conventional image processing device for forming an image on a paper, reproducibility of the device may become different between a main scanning direction that is orthogonal to a paper feeding direction and a subscanning direction that is orthogonal to the main direction due to its characteristics. When performing the screening process to an image using the FM screen and reproducing the image on a paper, a region may be generated where collapse or missing (friar) of dots occurs widely due to the difference of reproducibility between the main direction and the subscanning direction.
For example, an image processing device of an electro-photography system, in which a laser beam is scanned using a polygon mirror, etc. in a main scanning direction to form an electrostatic latent image on a photoconductive drum and the latent image is developed by adhering toner, has a tendency of higher reproducibility of dots in the subscanning direction than the main scanning direction. FIG. 12 shows an example of dots reproducibility of an electro-photographic image processing device. The figure has a size of 128×128 pixels and the dots are reproduced in 50% of tone value using threshold matrix for an FM screen having spatial frequency characteristics whose frequency domain is circular. In FIG. 12, as shown by region A, the dots tend to collapse by a toner when the figure contains main components along the main scanning direction (fine horizontal lines, for example). On the other hand, the dots do not tend to collapse by a toner when the figure contains main components along the subscanning direction (fine vertical lines, for example), but tend to generate white patch.
In the case where an image processing device having such characteristics is used, a screen pattern having a region P where dots are collapsed and a region Q where dots are missing, as shown in FIG. 13, is repeatedly arranged like tiles even when an image is processed by threshold matrix for the FM screen having no periodicity as an image data. As a result, the collapse and missing of dots are repeatedly reproduced, as shown in FIG. 14, and the reproduced image in such a way is recognized as moiré fringes which are an undesirable pattern having periodicity.
In relation to the above mentioned problem, a technique for making conventional threshold matrixes is disclosed is disclosed in JP2006-311532A. In the document 1, FM screen threshold matrixes that have spatial frequency characteristics whose frequency domains are elliptical and the directions of the major axes of the elliptical figures of colors differ from each other are generated so as to reduce graininess in a color image.